Various types of electronic test equipment use hand held probes to establish an electrical connection between the test equipment and a work piece. Sometimes these probes are passive; that is, they contain no active elements, such as FETs or bipolar transistors. Absent some special consideration, passive probes can generally be small, and their size is set by what is comfortable to handle, on the one hand, and functional in terms of electrical performance, on the other. (There are exceptions, of course, such as the foot long 30 KV high voltage probe for a voltmeter.) Active probes are necessitated by particular electrical considerations, generally involving high frequency operation and impedance. So, for example, an oscilloscope probe that is expected to work into the several gigahertz region needs to prevent the length of cable connecting the probe to the test equipment from being seen by the work circuit and must also exercise rigorous control over the stray reactances that the probe is allowed to bring into contact with the work circuit. The former requirement necessitates amplification, while the latter generally means keeping the actual contact leads short, sometimes very short, and locating an impedance conversion amplifier very close to the point of measurement. Hence the term ‘active probe’ arises; the impedance conversion (and other functions, too) are performed by significant collections of high performance active circuitry (typically a ‘hybrid circuit’) located in the probe itself. A hybrid circuit is often a small ceramic substrate with printed conductors and components mounted thereon, usually including one or more integrated circuit dies (shorn of their usual plastic encapsulation).
Some high performance active probes dissipate a modest amount of power to accomplish their tasks; hence, they have to shed heat through surface area. They are, by their very nature, ‘electrically exposed,’ as it were, and are not infrequently through ignorance, carelessness or plain bad luck, damaged by contact with excessive voltages. They can also be relatively expensive as small things go (say, on the order of a thousand dollars), and it is common for them to be returned to the factory for repair (vendors will sometimes offer loaners to the unfortunate owners during the interim . . . ). All of this means that active probes tend to be bigger than their passive brethren; you can generally always spot one because of its fat cable and its portly appearance. See FIG. 1.
The forgoing does not mean that presently available active scope probes and other active probes do not electrically perform as intended. But it sometimes happens, and given present trends in assembly design, with increasing frequency, that the physical size of the probe prevents it from being brought as close as desired to a point of contact within the work circuit. Consider a number of printed circuit boards plugged into a mother board, so as to all be parallel, one next to the other and spaced a minimum distance apart. It is necessary to probe a location at or near the middle of one of the interior boards. It often happens that the probe simply won't fit into the available space, and circuit performance (or lack of space) precludes the use of an extender board. A common solution is this situation is, with the board removed, to solder or otherwise attach a needed length of lead (or leads, if the probe is differential, and we mustn't forget about a possible ground lead) to the probe and to the work circuit, replace the board, and then make the measurement with the probe near, but outside, the space occupied by the boards.
There are some disadvantages to this. The leads tend to be made afresh each time to fit the circumstances, and they are almost always longer than desired (which would probably be ‘as short as practical’). The result is a nagging uncertainty in the quality of the measurement: what would the measurement have been if those leads were truly of minimum length?
One thing that can be done to ameliorate this situation is to make the body of the active probe thin enough that it can fit between the boards. Then the leads can be no longer than absolutely necessary. We may still need to solder some short leads that extend from the probe to the point of electrical contact, simply because of the geometry of the parts involved: there is no way to clip or hook the probe tip to the circuit (e.g., grab onto a component leg), or even if there is, it won't work because the probe will not both stay hooked and lay flat against the board to occupy the available space between the boards. But if the probe is thin enough, and will go into that available space, then we can expect that some very short leads manufactured for the purpose, will satisfactorily perform the interconnection task. What is more, there may well be a performance advantage in that the probe's electrical behavior is more repeatable and readily characterized when it always has the same type and length of flexible extension.
So, we set out to make a not-too-long probe that trades an increase in width for a drastic decrease in thickness. The innards of the probe can be a suitable hybrid circuit fabricated on a ceramic substrate. The departure from a circular cross section means that there will be plenty of surface area to dissipate heat, and it is not so small that it cannot be actually hand held if need be. (The probe even has tiny connectors in the front so that the flying leads can be disconnected and replaced with a different set of flexible leads or with rigid prongs, if desired.)
Now, in some ways an active probe is just like any other electronic apparatus. It has a chassis of some sort, that carries one or more assemblies that have the actual parts. It also has an outer housing that both mechanically and electrically protect the insides from what's one the outside (and probably vice versa, as well). Probes have cables that lead back to the test equipment they are associated with, and something has to anchor that cable to the probe body. It is not unreasonable to expect a probe housing to engage a boot or strain relief molded onto the probe end of the probe cable.
Given that the probe IS NOT a throw-away item, the housing needs to be something that can perform its protective (and anchoring) duties without preventing successful recovery of the innards. It also needs to be inexpensive and reliable, not to mention, simpler is better, as is fewer parts. No messy glues should be needed that could migrate to the interior and interfere with later disassembly. Tiny threaded fasteners come to mind, but they consume space on the part, are fussy and add cost, not to mention aggravation (you heard that little screw hit the floor, but now it's GONE!). Interlocking plastic pieces come naturally to mind. They can be tough, strong, insulating, and molded into particular shapes. But whatever it is, it needs to be thin, so as to not add any extra unnecessary thickness to the finished probe body. And for aesthetic reasons, as well as to discourage (or at least make evident) tampering with parts that may be under warranty (“but we can't fix it if you tried to fix it first . . . ”), the exact manner of how the protective cover pieces join themselves should be covered by a single use (or tamper-evident) adhesive label (which can also carry the manufacturer's logo and the probe's model number, etc.). None of these wished-for properties is an unusual one, considered in isolation. But we really need to satisfy the list with a housing that lets the active probe remain THIN!
What to do?